m2kbug:
I can't stand being tickled. I'm not terribly ticklish, so attempts at tickling takes more effort, but my feet are very ticklish. I've just firmly said "don't tickle me" or "I don't like to be tickled." Then "don't" or "stop," but repeated attempts just escalates quickly into an angry "stop." I don't know if this is necessarily rude. I have a very low tolerance or breaking point on this one. What is it about people and tickling? It's like the big red button people can't resist pushing. I guess when you see them reaching in your direction, back away and say, "Don't try to tickle me, I don't want to be tickled" and be very firm. If someone mentions your ticklishness in conversation (and you know they want to tickle you), you can say, again, "Don't try and tickle me." If they continue, and you get mad, well, that's kind of a consequence. "I told you not to and you did it anyway. Of course I'm mad." Attempting to stop the action before it starts is the best way. What you've been doing sounds perfectly acceptable to me.

amylouky:
I generally tell people, "I'd prefer you didn't tickle me but if you insist, know that you do so at your own risk.. I'm not taking any responsibility for any bodily injury I might do if you tickle me."Seriously. I'm probably moderate-ticklish, but I do react violently and completely involuntarily to being tickled. DH knows this and doesn't even try, after I hit him in a very uncomfortable place once by accident.

gorplady:
I'm confused. Are these random people that start to tickle you or people that know you?

Regardless, no one should touch you without your consent, which includes tickling.

weeblewobble:
My sister, while not quite as tickle-sensitive as you, does NOT like to be tickled or touched without permission. Particularly if that person sneaks up behind her in order to tickle or poke her. She suffered some sexual harassment in middle school, that we didn't find out about until later. I wouldn't say she has PTSD, but it's definitely as very strong aversion to this behavior as a result of her experience.

She was working at a children's summer program when we were still in college and she mentioned this aversion during a staff building exercise in which they were asked to share their pet peeves. (She realizes now this was a huge mistake and never mentions it to people.) One of the male staff members took this as a cue to sneak up behind her and "sneak attack" poke her in the side or tickle her. He got a big kick out of the way she jumped and flailed her arms when he did it and told her to "lighten up!" when she told him to cut it out. This continued for several weeks. The program director told the guy to keep his hands to himself, but didn't do much else.

Finally, one morning, this guy poked my sister in the side as she was turning around, she threw an arm up and her elbow caught him right in the nose. Cue a huge nasty nosebleed and the guy whining about Sis "hitting" him. Director told him he'd suffered the consequences of his actions and should learn a lesson. He cut Sis a wide path (and dirty looks) for the rest of the summer.

Now, I'm not encouraging you to actively seek opportunities to smack the imbeciles who are doing this to you. But I definitely wouldn't protect them from the consequences of their actions. You said you tend to flail about when tickled. Don't restrain this response, even if they are within range. If they get hit because they intentionally startled you and you reacted, that's their fault. Not yours. Maybe if there are negative consequences to their actions, they will stop.

peach2play:
IIRC a tickle sensation is a pain sensation. Use that, try and change your laughter to screaming and do not hesitate to get extremely upset with someone who is touching you with out your permission esp after they have been told not to. It is not ok and when they claim you are too sensitive, they are using bully tactics to get you to submit to their touch. You might also consider working with a behavioral therapist to help you change your reaction to being poked and tickled.